Moved ActivityDisplay to its own class file.

Clean-up before the next windowing mode refactor.
Also:
- Change the return type of ActivityStack.shouldBeVisible from int
to boolean since it is now binary.
- Removed ActivityStack.mRecentTasks. Changed call points to use
the reference in ActivityStackSuperior.
- Removed ActivityStack.mStacks and ActivityStackSuperior.mStack.
Changed call points to use reference in ActivityDisplay.
- Removed a few unused methods and variables.

Fixes: 66452053
Test: Existing tests pass.
Change-Id: Id9f4c668cdfc638820ad1777db85ac4d2619b85a